One application of the subject invention is directed to metal oxide electrodes for use in electrochemical cells. The battery has become a primary power source for many portable electronic devices such as cameras, radios, hearing aids, watches, calculators and the like. In order to maintain the overall electronic device as compact as possible, many electronic devices are designed with cavities to accommodate miniature cells as their source of power. The cavities are usually made so that a cell can be snugly positioned therein, thus making electronic contact with appropriate terminals within the device. A major potential problem in the use of a high energy density cell such as a divalent silver oxide/zinc/alkaline cell is that if the cell bulges it usually becomes wedged within the cavity of the device which sometimes can result in damage to the device. In addition, when the cell bulges it may disturb the seal whereupon the electrolyte might escape to cause damage to the device and/or oxygen from the atmosphere may enter which could cause wasteful corrosion of the negative electrode. On the other hand, if the seal of the cell is maintained, high internal gas pressure may develop which could cause not only bulging of the cell but even possible disassembly of the cell.
Silver oxide is a poor conductor, having a high resistance of greater than 20 megohm-in/in.sup.2. In the past it was advisable to add an electroconductive ingredient, such as graphite, to a silver oxide electrode. Graphite is a good conductive material and therefore it was used to enhance the conductivity of the silver oxide electrode. The addition of graphite will replace some of the active material of the silver oxide electrode, and for a fixed volume cell the capacity of the cell is decreased. It has been found that chemically inert electrical conductors, such as graphite, can be eliminated from a silver oxide electrode if a layer of silver is formed on the silver oxide electrode. Several U. S. patents have addressed the issue of how to reduce silver oxide in order to create a layer of silver on the silver oxide electrode.
U.S. Pat. No. 3,055,964 discloses the use of a heat treatment for electrodes containing divalent silver oxide and monovalent silver oxide so that the electrode will discharge at a constant voltage.
U.S. Pat. No. 4,009,056 discloses a primary alkaline cell having a stable divalent silver oxide depolarizer mix comprising a negative electrode (anode), a divalent silver oxide (AgO) depolarizer mix, a separator between said negative electrode and depolarizer mix, and an alkaline electrolyte. The surface of the depolarizer mix is treated with a mild reducing solution to form a reduced layer surrounding the mix, and the surface of the reduced layer adjacent to the separator is coated with a layer of silver.
U.S. Pat. No. 4,015,056 discloses a method for manufacturing a stable divalent silver oxide depolarizer mix wherein the mix is treated with a mild reducing solution of a reducing agent, such as methanol, followed by a treatment with a strong reducing solution of a reducing agent, such as hydrazine, to form a layer of silver on the surface of the depolarizer mix.
U.S. Pat. No. 4,021,598 discloses a zinc-silver oxide dry cell which comprises a zinc anode, a silver oxide cathode, an immobilized body interposed between the anode and the cathode, and a metallic silver layer formed at least on the surface of the anode side of the cathode in which the metallic silver layer is formed through a reduction reaction resulting from a light exposed to the surface of the cathode.
U.S. Pat. No. 4,048,405 discloses a silver oxide cell whose cathode comprises a mixture of monovalent silver oxide and divalent silver oxide and wherein a portion of the cathode is treated with a strong reducing agent to reduce the surface of the cathode to form a silver layer.
U.S. Pat. No. 4,068,049 discloses a zinc-silver oxide dry cell which comprises a zinc anode, a silver oxide cathode, an immobilized body interposed between the anode and the cathode, and a metallic silver layer formed at least on the anode side surface of the cathode by reduction of silver oxide. Reduction of the silver oxide to form the metallic cathode layer on the cathode surface is effected by causing a reducing agent to contact with the cathode surface or by subjecting the cathode surface to a heat treatment, electrolysis or a mechanical force.
U.S. Pat. No. 4,209,578 discloses an Ag layer that is formed upon an Ag.sub.2 O electrode by introducing a chemical reducing agent in such a manner that the transformation takes place after the electrode is already in place in the cell cup.
It is known in the art that plasma spraying is a line-of-sight process involving the injection of a substance, such as a powder, into a high temperature plasma jet. Essentially, the plasma torch or generator comprises an anode, such as copper, and a cathode, such as tungsten. A gas, such as argon, flows around the cathode and through the anode which is shaped as a constricting nozzle. An arc is maintained between the cathode and anode which ionizes the gas and thereby produces a plasma stream that exits the torch by way of the nozzle. Generally, a substance, such as a powder, is fed into the plasma jet stream and it is the powder that is then deposited on a substrate placed in the line-of-sight of the nozzle of the plasma torch. This process of plasma deposition or glow-discharge deposition is known in the art and is described in Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 10 at pages 262-265, John Wiley & Sons (3.sup.rd Ed. 1980).
It is an object of the present invention to provide a treatment for reducing the surface of a metal oxide substrate to form a layer of its metal on the metal oxide substrate using a plasma gas excited by a plasma generator.
It is another object of the present invention to provide an inexpensive plasma treatment for reducing the surface of a metal oxide substrate to produce a layer of its metal on the surface of the metal oxide substrate.
It is another object of the present invention to provide a treatment for reducing the surface of a metal oxide electrode, such as silver oxide electrode, to form a layer of the metal, such as silver, on the electrode using a plasma gas excited by a plasma generator.
The foregoing and additional objects will become more fully apparent from the following description.